Xerographic printing employing raster output scanners (ROS) has evolved along the lines of greater efficiency, higher data throughput rates, improved print quality, lower cost of manufacture, and smaller packaging. These metrics of improved xerographic printing are influenced by various factors. The goal of designing a ROS system is to choose some combination of factors that produces a system that optimizes these metrics.
Efficiency of a ROS system has, in general, two major components: power throughput efficiency and scanning efficiency. Power throughput efficiency is defined as the ratio of the power output of the laser to the power of the beam received at the photoreceptor, as calculated in a "non-dynamic" mode (i.e. no polygon movement). Primarily, power throughput efficiency is a function of the type of illumination the facet receives and of optical element surface losses.
Optical element surface loss is due to undesirable absorption, reflection, and transmission in the various optical elements. While these factors can be controlled to some extent, there comes a point at which no further efficiencies may be achieved at the optical element level. Instead, there are certain system design choices that can be made with the goal of maximizing scanning efficiency.
Three methods of polygon facet illumination are currently practiced in ROS systems: over-filled, under-filled, and facet tracked. As regards efficiency, facet tracking has certain advantages over the other two designs. For example, facet tracking avoids the power throughput loss of over-filled scanning while nearly adopting its perfect scanning efficiency. Likewise, facet tracking avoids the scanning inefficiencies of under-filled scanning while adopting its high power throughput. Thus, facet tracking is generally a better choice of scanning techniques when designing an optimally efficient system.
As mentioned above, another metric of xerographic printing is data throughput. To improve data throughput rates, many present systems employ multiple laser beams to concurrently "write" on the surface of a photoreceptor. The speed-up of the printing process is generally proportional to the number of independently addressable beams writing on the photoreceptor.
One prior art system employing multiple, independently addressable beams in a facet tracked configuration is detailed in sagittal view in FIG. 1. System 10 employs a gas laser 12 as its light source. A pre-modulator lens 14 provides the correct shape of the beam for beam splitter 16a which then splits the input beam into multiple beams (e.g. beams 18a and 18b). The acoustic-optic modulator cell 20 provides multi-channel modulation (e.g. channel 22a and 22b) for the multiple beams.
As will be discussed in greater detail below, A-O cell 20 accomplishes two functions: it modulates the light according to an input video signal and deflects the beams to maintain facet tracking. The output beams from the A-O cell are recombined in beam combiner 16b, transmitted through post modulation optics 24, and reflected by a mirrored facet on a rotating polygon 26. The reflected beams 28 are transmitted through the focusing and polygon wobble correcting optics 30 and imaged onto the surface of photoreceptor 32.
FIG. 2 shows the workings of the A-O cell 20 in greater detail. In order to provide multi-channel modulation, the A-O cell is partitioned into regions (e.g. regions 44 and 46), allowing the various beams (e.g. beams 18a and 18b) to be independently addressable.
To create the partitioning for the different optical channels, individual transducers (e.g. transducers 34 and 36) are coupled to A-O cell. Each transducer transmits the sound wave (e.g. wave 42) that propagates through the channel. As is well known, the frequency of the sound wave determines the angle of deflection of the light beam.
The sound wave for a given channel is generated by a separate driver (e.g. drivers 38 and 40). The drivers accept their own digital video data input in order to modulate the light beams. As mentioned above, the modulation of separate channels by corresponding drivers allows each beam to be individually addressable.
The system, depicted in FIGS. 1 and 2, is not optimal from the standpoint of several metrics. For example, the use of beam splitters and separate acoustic channels operated by dedicated drivers adds greatly to the cost of system manufacture. Moreover, the use of a gas laser as the light source increases the packaging size of the system. For example, gas lasers generally have a minimum size of one inch in diameter and 6 to 8 inches in length. These dimensions represent a lower bound on the size of a optical system employing gas lasers.
Thus, there is a need for a multiple, independently addressable beam ROS system that is not as complex or costly to manufacture. Additionally, there is a need for a multiple beam ROS system that has less of a space requirement than current systems.
It is therefore an object of the present invention to provide a system that employs a multiple, independently addressable beam ROS system that does not require beam splitting and recombining.
It is another object of the present invention to provide a multiple beam, facet tracking ROS system having a lower cost of manufacture.
It is yet another object of the present invention to provide a multi-beam, facet tracked ROS system having a reduced packaging size for the system optics.
It is another object of the present invention to provide a system that employs a method of modulation having faster switching characteristics than a typical A-O cell.